The present invention relates to a method for essentially identical plasma-enhanced chemical coating of opposing surface areas of a workpiece, an apparatus or facility for carrying out the method as well as applications of the method or the facility.
Plasma-enhanced vapor deposition methods are known as are facilities used for this purpose. In this connection reference is made to "Plasma deposition of inorganic thin films", Alan R. Reinberg, Ann. Rev. Mater. Sci. 1979, pages 341-372.
To generate a plasma it is also known to generate an electric field between two electrodes, for supplying the required energy to a gas mixture in a vacuum chamber, for the formation of the plasma. The space between the electrodes can basically be divided into two dark regions immediately in the vicinity of the electrodes with the plasma region located between the dark regions. While in the plasma region a high density of electric charge carriers is present, the space charge density resulting from the charge balance per unit volume is zero, the dark regions have a definite space charge density.
Due to the non-vanishing charge balance in the dark regions the plasma space assumes the plasma potential. Within the plasma region the potential is essentially constant. To the specialist the plasma region is a clearly defined space in connection with which reference is made for example to "Glow Discharge Processes", Brian Chapman, John Wiley & Sons (1980), pages 77-80.
In coating workpieces with the above cited method which is called PECVD (plasma-enhanced chemical vapor deposition), it is customary to dispose the workpiece on one of the electrodes. For coating with conducting layers the electrodes are operated with dc or ac voltage while for coating with insulating materials an ac voltage is provided.
A PECVD method with appropriate configuration is known from U.S. Pat. No. 3,847,652 for fabricating osmosis membranes. The plasma streams from one side against a carrier on which the membrane is to be coated while a monomer gas is supplied from the opposite side. The plasma generation takes place by means of high frequency energy on an electrode pair at a distance from the carrier. U.S. Pat. No. 3,847,652 is incorporated here by reference.
The coating of the membrane is formed specifically on one side of the carrier plate and an explanation is provided on which surface is better suited for the coating, given different surface formations of the carrier.
Furthermore it is known from German Patent DE P 33 21 906 to coat metallic powder with a synthetic film through plasma-enhanced chemical vapor deposition by stirring the powder in a rotating vacuum container between electrodes for the generation of the plasma, whereby the powder is coated on all sides.
As has been stated it is customary in plasma-enhanced chemical vapor deposition to position workpieces to be coated, which in contrast to powder cannot be stirred in the process chamber, on one of the electrodes. The disadvantage is that the workpieces are coated only from one side and that consequently subsequent turning is necessary if they are to be coated on both sides. Moreover, identical or uniform coating of workpieces of different height or of workpieces with arched surfaces is difficult as will be shown later. Three dimensional coating known according to German Patent DE P 33 21 906 for granules in which the granules are stirred back and forth due to the rotary movement of the reaction vessel in the plasma region and the dark regions, is clearly not possible for coating workpieces such as lenses, filter glasses etc.
From U.S. Pat. No. 4,361,595 it is known to generate a dc or ac discharge between two electrodes in a gas vacuum chamber. To form an abrasive layer of SiO.sub.x on synthetic disks by means of a PECVD method, in one implementation two synthetic disks to be coated are placed one on top of the other and introduced into the discharge space formed between the electrodes. U.S. Pat. No. 4,361,595 is incorporated here by reference.
It is known that only minimal requirements are made, if any at all, on the identity of abrasive layers on surfaces of different disks.